From time to time new materials are discovered that serve as a basis for new or improved technologies with substantial commercial value in the marketplace. In the present day, there is considerable interest in discovering new materials for the development of spin-transport electronics (“spintronics”), in which the spin of charge carriers is exploited to provide enhanced functionality for microelectronic devices. In particular, the development of room-temperature ferromagnetic semiconductors comprises a central part of an ongoing, intensive effort to develop spin-based field effect transistors, spin-based light emitting diodes, and magnetic random access memory. If suitable novel classes of materials can be developed to underpin these new devices, they would enable a revolution in electronics and information technologies.
Ferromagnetic semiconductors are possibly the most intensively studied materials for spintronic applications. GaAs and ZnO doped with 3d elements are now widely studied as prototypical ferromagnetic semiconductors. The ideal ferromagnetic semiconductor material preferably has the following basic characteristics at room temperature:                1) Semiconducting gap of order 0.1-0.3 eV;        2) Ferromagnetic state with an ordered moment of at least 0.05 μB/per formula unit;        3) Carrier concentration (1016-1018/cm3) sufficient to provide a useful spin current;        4) Carrier mobility on the order 10−2-10−4 cm2 V−1·s−1;        5) Moderate ferromagnetic coercive field (200-500 Oe) to facilitate switching of spin polarization; and        6) Compatibility with commercial substrate materials such as Si or Al2O3.        
One obstacle to the implementation of spin-polarized semiconductor devices is the absence of suitable room-temperature, soft ferromagnetic semiconductors (FS's). Dilute magnetic semiconductors are under intense study for applications in spintronics. However, the weak solubility of randomly placed magnetic ions in the semiconductor host makes these materials unsuitable for devices. It is, therefore, desirable to develop a room-temperature FS based on a periodic array of magnetic ions.
We have found that ternary and other complex ruthenium ferrites exhibit long-range ferromagnetic order well above room temperature, accompanied by narrow-gap semiconducting properties that include a large anomalous Hall conductance, low resistivity, and high carrier concentration. Additionally, the physical properties can be tuned by simple chemical substitution of two elements, Fe and Co, or by varying the relative concentration of 3d and 4d elements within a homogeneity range that we have established. These promising properties—manifest within a single structural family—provide a fertile ground for fundamental studies and open up a host of potential device applications.
One inventive composition according to the present invention possesses a high ferromagnetic ordering temperature Tc that we have shown can be freely tuned from 300 to 500 K by doping. This material is semiconducting with an approximate gap of 200 meV. The coercive field at T=300 K is nearly ideal at Hc=275 Oe. X-ray data show that samples are single phase, and single crystals are obtainable. These initial results indicate that this material is a viable candidate for spintronic devices.
Further, we have synthesized another closely related metallic ferromagnet with extremely small coercive field (≈1 Oe) at T=80 K. This value is the same as for permalloy Ni81Fe19, which is a widely used metallic soft ferromagnet. It is particularly noteworthy that both of these inventive materials have the same crystal structure, which should facilitate the fabrication of composite heterostructures using these otherwise distinctly different materials.
We wish to emphasize that the new compositions come from a class of materials that are absolutely distinct from the GaAs and ZnO materials studied by other laboratories. The inventive compositions are not a “diluted magnetic semiconductors,” which usually suffer from clustering of magnetic ions among random lattice sites. Rather, the magnetic ions in these materials reside on a periodic lattice. Thus, we have discovered an entirely new paradigm for ferromagnetic semiconductors with the potential for widening this class into an ensemble of interesting, compatible materials having a range of physical properties with commercial potential.
Compositionally, the materials discussed above belong to a class of materials known as oxoruthenates. Oxoruthenates have attracted recent attention due to their electronic and magnetic properties. Advantageous properties of these materials include, for example, the occurrence of unconventional superconductivity, metamagnetism and itinerant ferromagnetism. A large number of ternary and multinary ruthenates can be described in perovskite structures and variants thereof. One general feature is the occurrence of octahedrally-coordinated Ru connected with additional tetrahedrally-coordinated metal species.
According to the present invention, large single crystals of strontium and/or barium ruthenates containing a further transition metal from the third period of the periodic table have been grown with the aim of elucidating the crystal structures and magnetic properties of these attractive compounds, with a particular focus on the interdependence of magnetic properties and chemical composition.
According to one embodiment, the invention relates to single crystal and/or polycrystal oxoruthenates having the generalized composition (Baz,Sr1−z)MxRu6−xO11 (1≦x≦5; 0≦z≦1; M=Fe, Co). According to a further embodiment, the inventive oxoruthenates have the generalized composition (Baz,Sr1−z)FexCoyRu6−(x+y)O11(1≦(x+y)≦5; 0≦z≦1; when z=1 then x≠0). In an embodiment, x=0, or y=0, and when x=0then z≠1. In another embodiment, the composition has a Curie temperature greater than or equal to about 300 K.
According to yet a further embodiment, the invention relates to single crystal oxoruthenates having a composition BaFe2+xRu4−xO11 (x=1.4) and BaCo2−xRu4+xO11 (x=0.2). The invention also relates to a broader range of ternary compositions, SrM2±xRu4∓xO11 (M=Fe, Co) and (Ba,Sr)M2±xRu4∓xO11(M=Fe, Co).
Black plate-like single crystals of quaternary transition metal oxoruthenates having the composition BaFe3.39(5)Ru2.61(5)O11 and BaCo1.85(6)Ru4.15(6)O11 (hexagonal, space group P63/mmc (No. 194), Fe: a=5.856(1), c=13.587(1) Å, R1=0.029, wR2=0.084; Co: a=5.842(1), c=13.573(3) Å, R1=0.033, wR2=0.075) have been grown from a BaCl2 flux.
X-ray refinements and charge balance considerations suggest that Co2+ and mixed valence state Ru3+/Ru5+ and Fe2+/Fe3+ are present in these compounds. Different occupations of the M(1) and M(2) sites by Ru and the 3d elements lead to deviations from the ideal compositions, BaM2Ru4O11 and SrM2Ru4O11; therefore, a homogeneity range (BaM2±xRu4∓xO11 and SrM2±xRu4∓xO11) has been discovered, which has important effects on physical properties. In one embodiment, the invention relates to a composition represented by the general formula BaM2±nRu4∓nO11,wherein M=Fe and 0<n≦1.4, or Sr M2±nRu4∓nO11, wherein M=Fe or Co and 0≦n≦1.4. Crystals grown from a BaCl2 flux and/or a SrCl2 flux according to the present invention are not restricted to a fixed composition.
Polycrystals of SrFe3Ru3O11 or single crystals of SrFe2.6Ru3.4O11, SrFe2.8Ru3.2O11, and SrCo2Ru4O11 were synthesized by solid-state reaction or grown from SrCl2 flux. These compositions were confirmed by single crystal X-ray refinement and micro-probe analyses. Powder Rietveld refinement of the X-ray diffraction data were consistent with a composition of SrFe3.0Ru3.0O11 (a=5.8375(4), c=13.403(1) Å).
The crystal structures contain two crystallographic sites with mixed Fe/Co and Ru occupation of different levels in octahedral coordination, and one site purely occupied by the respective 3d-metal. The latter position is in trigonal bipyramidal coordination, with some indication of a displacement of the metal cation towards tetrahedral coordination. According to the charge balance, the ruthenium is incorporated with different electronic situations in the two Ru-containing sites. The Co compound may be described as containing Co2+ and Ru5+ next to Ru3+Magnetic susceptibility data support this assignment.
According to magnetization measurements on orientated crystals, BaCo1.85(6)Ru4.15(6)O11 is a soft ferromagnetic material with low coercive field and a spontaneous magnetization below Tc=105 K. It behaves as an electric conductor. However, BaFe3.39(5)Ru2.61(5)O11 is a narrow band semiconductor material with ferrimagnetic ordering at Tc=440 K.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.